Sensor noise is a common problem encountered in analog and digital control systems. In most situations, the noise must be reduced or, if possible, eliminated. In filtering sensor output signals, it would be desirable to detect changes in the sensor signal and adjust the noise filtering accordingly. For example, it would be desirable to adjust the filter such that the filter time constants do not detrimentally effect the sensor output during desirable changes in the sensor input.
In one common approach, noise is reduced by incorporating hysteresis in a feedback loop. Hysteresis can substantially reduce or eliminate noise. However, if noise spikes are of significant amplitude, hysteresis may decrease static accuracy and add delay to the system in the frequency range of operation. Hysteresis can also induce limit cycles. Including hysteresis in the feedback may also involve incorporating non-linear elements, and non-linearities may degrade system performance. To minimize detrimental effects, the hysteresis halfwidth may be chosen to eliminate most of the noise. However, with reduced hysteresis halfwidth, noise spikes greater than the halfwidth will pass through the filter, at a reduced amplitude.
Another common approach to noise reduction is to filter the noisy signal with a first or higher order filter. Filters do not normally possess the undesirable characteristics of a hysteresis. However, if the noise peaks are of significant amplitude, the time constant(s) required to provide sufficient noise attenuation may adversely impact transient response.